1. Field of the Invention
The present invention relates to an image forming apparatus such as a copy machine, a printer or the like which provides at least two developing means.
2. Description of the Prior Art
A copy machine having plural developing units being settable thereinto has been well known. In such a copy machine, each developing unit provides a signal means for identifying a color of toner contained therein and, due to the signal means, each color of toner of respective developing units having been set in the copy machine can be detected. When a desired color is designated upon copying, a developing unit having the desired color of toner is automatically selected to develop an electrostatic latent image of a document formed on a photoconductor drum with the toner of desired color.
In such a multi-color copy machine, the black developing unit, namely the developing unit containing black toner is assigned as the standard one to be equipped in each copy machine since most of documents to be copied are black and white. If a copy machine has an upper and a lower developing stages, the black developing unit is usually set at the lower developing stage as the standard one and another developing unit is interchangeably set at the upper developing stage.
Assuming that two developing units of the same color such as black are set at the upper and lower stages, respectively, the amount of consumption of toner of the lower developing unit becomes much more than that of the upper one since the frequency in use of the former is higher than that of the latter. Accordingly, when the composite copy of two colors is repeated after the upper one has been exchanged to another developing unit of a different color, the lower developing unit is apt to become into a state of toner empty which requests to replenish with toner. This is inconvenient for the user and is applied to the case of an image forming apparatus other than the copy machine which provides plural developing means.